This invention relates to an improved method of forming a seamed metal tube having a metal coating. More particularly, the present invention relates to a process suitable for continuous or in-line processes for forming ferrous metal tubes coated with a protective metal coating, preferably, zinc, aluminum or copper and their alloys. It should be understood that the process can be applied to non-ferrous metal tubes, including, for example, brass.
Methods of continuously or in-line forming of a seamed steel tube from a continuous strip or skelp are well known. In a conventional tube forming mill, the continuous strip is first cleaned and conditioned, then rolled to form an open seam tube having nearly abutting edges at the top of the tube. The edges are then welded together by one of several conventional methods which generally include heating the edges and then either forging the edges together with squeeze rolls and/or flux welding the seam. The edges of the tube may be heated, for example by resistance welding, electric arc or by high frequency induction welding. High frequency induction welding is a form of electric resistance welding, wherein the open seam tube is received through an electric work coil which creates a strong magnetic field, which in turn induces a current to flow around the tube and in the "Vee" formed as the edges of the strip are welded. An impeder is generally located within the tube, which forces the current down the nearly abutting edges of the open seam tube, heating the tube edges to a hot forging temperature. The tube edges are then forged by squeeze rolls which drive the molten edges together to form an integral seam.
In-line galvanizing and coating or painting processes are also well known. The strip or skelp may be galvanized or painted on one or both sides prior to forming and welding, or the outside surface of the welded seamed tube may be galvanized by immersing the tube in a molten zinc bath. Where the strip is coated with a protective coating prior to seam welding, the coating will burn off or melt in the seam zone because the welding operation involves the melting of the tube material, which is generally steel. Thus, the temperature at the seam may be 2,300.degree. F., or greater. Where the strip is coated with a metal, such as zinc or aluminum, the metal will melt during welding and flow downwardly away from the seam, which is located at the top of the tube. A zinc coating solution has also been used to paint the exterior surface of the seam. However, such coating have poor adherence and are mainly cosmetic. "Metalizing" of the seam surface has also been attempted; such coatings are mainly mechanical and do not provide a metallurgical bond between the metals. The failure of the prior commercial processes to fully coat and thus protect the tube seam is evident by the fact that the weld area is generally the first to fail in accelerated corrosion tests. Metalized surfaces covering the exterior seam surface have been known to fail while incurring mechanical stresses associated with tube fabrication.
The continuous tube forming process and apparatus of this invention solves the above-identified problems and produces a superior metal coated tube. The process of this invention assures a fully-coated welded seam without substantial additional costs and may be used with conventional tube forming processes and equipment.